it says in the character analysis that krogstad was shunned by society and wasn't let by people to move on from his past. i think that because of this, krogstad tries to blackmail nora for her forgery as a means of compensating for the unfair treatment he received.

In our Lit class we also discussed the hypocritical nature of Torvald, and how he goes directly against what he earlier states are his attitudes and how he would respond (for example, he says "I am not so heartless as to condemn a man... because of a single false step", yet he is quick to condemn Nora when he discovers the forgery she had committed).